Island Foxes and Carrying Capacity

"Carrying capacity" is a term used in biology to describe when a habitat has reached the maximum population of a specific species that can be supported by the habitat's resources.

Think of a table with a platter of cookies. If there are more cookies than people, individuals will recruit others to enjoy the cookies; the population at the table increases. If there is just one cookie for each person, carrying capacity has been reached. If there are more people than cookies the table has exceeded its carrying capacity. People who need cookies will have to leave the table and go to another habitat; there aren't enough resources to support the larger human population.

Seeing three island foxes in such close proximity, as in the image above, is unusual.  A unique weather situation provided unusual resources.

Because island foxes live on islands, they are definitively confined to the resources of their habitats. They can not migrate to another location when resources or climate become challenging. Carrying capacity, however, isn't a strict number. The Channel Islands are wild natural places and island resources vary from year-to-year depending on rainfall and weather. 

Island foxes on Santa Rosa Island (pink line, in graph) have been increasing as they recovered from near extinction twenty-two years ago. In 2021, biologists documented the first estimated decrease in the Santa Rosa Island fox population. (Official 2021 population numbers will be available later this month.) This could be signally that the population estimate of 2,657 in 2020 was a larger population than the island's resources could actually support.

How does the population adjust down? Older island foxes (8–10 years old) may have a more difficult time surviving. They may be pushed out of their established territory by younger foxes. 

In addition, reproduction can slow down, decreasing the natural population replacement of individuals. We know that very few pups were documented on Santa Rosa Island in 2021. 

This female fox on Santa Rosa Island (right), identified as F257, was captured in 2019 as pup and then again in 2020 and 2021. At two years old, F257 is in her breeding prime, but she has yet to produce pups. This most likely is in response to limited resources.

Santa Rosa Island was extremely dry in 2021. Canines are adapted to produce offspring in balance with the amount of available resources. A female island fox with lots of food resources can produce five pups in her annual litter, but if there aren't enough resources or available territorial space to keep pups safe, her body can reabsorb embryos and give birth to no pups. 

Carrying capacity can sound ominous, but it doesn't mean that island foxes will experience a die-off event because they have reached a specific population number. 

If we look at Catalina Island (red line, in graph above), we can see that the island foxes on Catalina reached carrying capacity around 2013–2014. A healthy population on this island appears to be between 1,800 and 2,200 island foxes. During years of normal rainfall, resources increase and so do island foxes. Drought years, like the winter of 2015–2016, can result in natural fox fatalities and less reproduction. The population gradually adjusts down. When normal rainfall returns, the population rebounds. This is the natural fluctuation expected in a wild population. 

Now that the Santa Rosa Island fox population is at a high and has potentially reached carrying capacity, 2022 is the perfect time to re-evaluate territory-size and fox movement to access various island resources. Collecting GPS tracking data this summer will provide valuable data on territory, the current interactions between individual foxes, and how they access resources in their habitat.

Your donations are helping fund two important GPS-tracking studies on Santa Rosa Island this year.

FIF is also taking applications for the FIF 2022 Research Grant.

The Science of Counting Island Foxes

courtesy of National Park Service

In 2000 when there were only 15 island foxes surviving in captive breeding pens on San Miguel, you could count them easily on your fingers.

Today all Channel Island foxes across the six islands where they live are back in the wild. (A few rescued individuals from San Clemente Island can be seen in mainland zoos.) The recovery of the endangered island fox on San Miguel, Santa Rosa, Santa Cruz and Santa Catalina Islands has been record-breaking and the days of counting all of the physical island foxes are long gone.
 

courtesy of National Park Service

The process of counting foxes on each island begins in the fall with special capture cages.

These cages are set up either at the cross sections of grids or along a string depending on the island and its topography (More on strings and grids)

Cages in an area are set out over the course of a specific number of nights and checked each morning. On Catalina, each string is run for four consecutive nights; after that, typically the majority of individuals would be foxes that had already been captured on previous nights. Each island fox is identifiable because it has a Passive Identification Transponder (PIT) microchip. An island fox caught for the first time receives a PIT tag. From that point on the PIT tag allows it to be recognized as a specific individual. It can take weeks for biologists to evaluate each island.

Over the course of all the capture nights, the number of individual island foxes, their gender, ages and location caught are documented. Island foxes are quite territorial, therefore it is unlikely that a fox caught in one area of the island would be found on a different area of the island during the month of counting. (If it is, that is important too.)

The data is input into a modeling program for analysis. Different programs may be used on different islands and over the years land managers may change which program they use. The modeling program calculates the density of island foxes in specific habitats across the island and estimates a total island population.

Sometimes, however, it takes a human brain to see complexity in the numbers. On two islands where recovery has been robust over the past few years, San Miguel and Santa Catalina, the estimated population numbers appear to be higher in 2013 than they were in 2012.

But observations on both islands conclude that drought may have reduced births and did cause starvation in a number of pups last summer, prior to counting. How could the population number increase while the number of surviving pups decreased significantly?

Biologists Calvin Duncan and Julie King from the Catalina Island Conservancy explain that on Catalina they caught a greater number of adult island foxes that had not been counted for a number of years. Young animals are more likely to be curious and challenged with finding food; they venture into capture cages more readily. Once an animal is in the cage, no other can be caught that night. This year, on the fourth night of capturing, they were still finding a high percentage of animals that had not been caught on previous nights.

courtesy of Kevin Schafer

The increase in the population number for Catalina is warranted because of the number of unique individual animals that were counted. But rather than an increase in the population, the number represents a refinement of the estimate toward greater accuracy.

On both San Miguel and Santa Catalina Islands, island fox numbers have surpassed historic population figures. Both islands may have reached carrying capacity, or the number of individual animals that can find the territory and food resources necessary for a healthy life. As the drought continues this summer, there may be further impacts not only on pups, but also on older individuals.

Counting island foxes provides an important picture of island fox recovery, but observations throughout the year, health checks, blood tests, data collected from radio-collared island foxes and necropsies is vitally important to understanding the whole recovery picture.

Counting Channel Island Foxes

How do you count a population of small animals across an island, when they are so well camouflaged?

How long did it take you to find the island fox?   photo courtesy of M. Jakubowski

Each July to December, across the six Channel Islands where island foxes live, biologists put out special capture cages to catch individual animals. These wire box-traps have a door that snaps shut when the fox steps on a trigger plate at the rear of the box.

The number of cages put out each night and the manner in which they are arranged differs between islands and land managers. Tim Coonan, senior biologist for Channel Islands National Park explains that on Santa Rosa and San Miguel "Traps are deployed in small grids." These grids typically are stretched over rolling hills. The grid pattern may be two traps by six traps wide or three traps by six traps wide. The capture-trap locations are spaced approximately 250 meters apart with the capture cages usually under bushes or scrub. The cage is covered with  burlap to further protect the fox from the elements.

Cages are baited with fragrant substances particularly interesting to island foxes. Each cage has food and water. The grid of cages is checked each morning. Foxes in the cages are first scanned for their microchip ID. Biologist Calvin Duncan explains why this is important.

Each individually identified island fox is counted. This year's pups are counted separately to determine an estimated fertility rate. Pups born in April are old enough to be on their own, but they usually are somewhat smaller than adults and will not yet have received an ID microchip. While in-hand, the island fox receives a health check, vaccinations and an ID microchip or radio tracking collar if needed.

On Catalina Island capture cages are put out along lines called "strings." On mountainous terrain, strings of cages are more practical than grids. On all of the islands, actual numbers of individual island foxes caught during the fall are entered into various population modeling software to determine the best estimate for the total population. After strenuous weeks of counting in the field, it takes months in the office to produce reliable numbers. That is why the number of island foxes counted in the fall becomes official in June of the following year.

These are the official island fox population numbers through fall of 2012.

• San Miguel Island - 540, up from a low of 15 in the year 2000
• Santa Rosa Island - 637, up from a low of 15 in the year 2000
• Santa Cruz Island - 1,354 , up from a low of ~62 in the year 2002
• Santa Catalina Island - 1,502 , up from a low of ~103 in 2000

Notice how Santa Catalina and San Miguel both showed slight population drops last year. Biologists believe this a sign the populations are stabilizing at their natural levels. Drought this year may impact populations as well. Notice too how Santa Rosa is finally showing robust population increase. We hope this population will continue to recover. Whether the numbers are up or down, counting island foxes is the only way we can have the best estimate of population recovery.

Island Fox Population Update

courtesy of Anita Machlis

Every year biologists count island foxes across the California Channel Islands in the late summer and early fall. Island foxes are captured in safe traps so they can be given health checks, fitted with radio collars and given vaccinations. When the biologist from the six different islands meet for the annual Island Fox Conference in June, they share their official population numbers and discuss the successes and challenges from each island.


The official population estimates from 2010 are:

• San Miguel Island - 516
• Santa Rosa Island - 292
• Santa Cruz Island - 1302
• Santa Catalina Island - 1008
• San Nicolas Island - ~500
• San Clemente Island - 1127

graph generated by Friends of the Island Fox 2011

The numbers show that island fox recovery has been very robust on San Miguel, Santa Cruz and Santa Catalina Islands. The island foxes on Santa Rosa Island are recovering, but at a much slower rate. Each island fox population has its own challenges. For the current status of island foxes by island Download - Island Fox Update 2011 pdf

Stay tuned for updates on 2011 population numbers as they come in from the autumn field counts. 

It's Pupping Season for Island Foxes

Across the Channel Islands a new generation of island foxes are being born. Each litter of pups are an important step toward full recovery for the endangered populations on San Miguel, Santa Rosa, Santa Cruz, and Santa Catalina Islands.


Island foxes reproduce only once a year, in the spring. This year's weather has been good for native plants which suggests we could see a large number of healthy island fox pups.

With small parents, island fox pups, or kits, are very tiny at birth. They weigh approximately the same as two AAA batteries and would easily fit in the palm of your hand.

Young foxes are very vulnerable and are born in a den. The den provides protection from weather and other animals for the first weeks of their lives. Here the mother feeds them milk until they start to eat solid foods. The father brings food back to the den both for his mate and the pups.

Unlike most other canines, island foxes don't appear to dig. They must find a den site either in a hole dug by another island species, (spotted skunk, ground squirrels, etc.) or a naturally made hollow under dense vegetation, tree roots or rocks.
Pups born in March or April are emerging from dens on the islands in May. Both parents care for the youngsters, helping to feed them and teaching them how to find food. This pup is with its father, an older male, in the fog on San Nicolas Island.

Island foxes grow up fast and by September they will be ready to leave their parents and strike out on their own. Annual counting of island foxes doesn't occur until the fall to insure that pups are weaned and somewhat independent. During trapping and counting, island foxes also undergo annual health checks. Radio monitoring collars are replaced or put on and island foxes are given vaccinations against distemper and rabies.

Last year biologists recorded impacts on island foxes from climate change. Continued drought reduced fruit on the islands' native plants. The combination of increased temperatures, low rainfall and reduced food resources caused the litter size born to each pair of island foxes to be small. A normal island fox litter is 2-3 pups. In good years, litters of up to 5 pups are possible. Unfortunately, the pups born in 2009 also had reduced birth weight, impacting their survival.

This year's normal rain levels may bring more positive pup numbers. Biologists are hoping for numerous healthy pups to continue the island fox's successful recovery.

Good News for Island Foxes

The numbers are gradually coming in from the biologists on the six Channel Islands that are home to the island fox. On the four islands where island foxes are endangered, San Miguel, Santa Rosa, Santa Cruz, and Santa Catalina, the populations continue to increase.

Each autumn, island foxes are counted by trapping them in specially designed containment traps. This allows biologists to determine the number of island foxes on each island, check their health, give them vaccinations for distemper and rabies, and fit radio tracking collars.

For more on Health Checks on Catalina Foxes.

Last year the number of island foxes physically counted on Santa Catalina Island was 365. This year over 500 have been individually counted!

Endangered island fox populations are recovering successfully on all four islands, but we still have a long way to go to reach the population numbers before disease struck Santa Catalina Island and ecosystem imbalance resulted in golden eagles hunting island foxes on the northern islands. Why are island foxes endangered?

Island fox recovery has been possible because of broad reaching conservation efforts on their behalf.

You can help !

Donations To Friends of the Island Fox Help:

• purchase radio collars to monitor wild island foxes. Learn More
  
• supply vaccinations against disease transmitted from domestic dogs and species transported onto the islands. Learn More
  
• fund necropsies to study the cause of death in island foxes. Learn More
  
• support education programs to teach school children and community groups about this local endangered species and the Channel Island Ecosystem. Learn More
  

You can Donate by clicking on the PayPal or Network for Good buttons at the top right corner.

Stay Linked In. As soon as the final population numbers are available, we will post them here on the Friends of the Island Fox website.

For detailed information on each island fox population as well as research summaries, video and podcasts - Click on the Library button in the directory bar.